1. Field of the Invention
The invention relates to a measuring device for measuring an electrical current having an alternating-current converter that possesses a primary inductance and an ohmic secondary resistance and whose output is connected to a measurement output for a current measuring signal across a measured-value signal conditioning device.
2. Description of Related Art
A measuring device of this type for measuring the output current of a half-bridge circuit is disclosed in DE 41 17 505 C2. The half-bridge circuit has two half-bridge sections connected in series that are connected to an output circuit at a connection node. A series circuit comprising a first semiconductor switch and a shunt is disposed in a first half-bridge section, and a second semiconductor switch is disposed in a second half-bridge section. The semiconductor switches are driven in alternation in push-pull operation by means of a control device. In this push-pull operation the first semiconductor switch is closed during a first operating state, and the second semiconductor switch is closed during a second operating state. In the first operating states, one measured value for the current flowing through the first semiconductor switch is acquired each time with the aid of the shunt. This measured value provides the measurement result for the output current. In the first operating states, a second measured value for the alternating current in the output circuit is also acquired by means of an alternating-current converter. This measured value is added to the first measured value. Then the result of this addition is inverted and saved. In the second operating state a measured value for the alternating current flowing in the output circuit is acquired and added to the saved value. The resulting total forms the measurement result in the second operating state.
The measuring device has proven to be effective in actual practice, mainly because it makes it possible to measure current in an output circuit that has a high electrical potential compared with the measurement circuit. However, a disadvantage of the measuring device is that the alternating-current converter has an ohmic secondary resistance at which the secondary current in the alternating-current converter causes a voltage drop. This voltage drop is present at the primary inductance of the alternating-current converter and causes current to flow in this inductance. As a result, the secondary current is no longer proportional to the primary current. As the frequency of the primary current decreases, the share of this current in the primary inductance increases. In order to achieve a low cutoff frequency, the primary inductance of the alternating-current converter must be sufficiently large. This results in the alternating-current converter having a larger volume.